The present invention relates in general to coating methods for metals and, in particular, to a new and useful method for chromizing ferrous-base and/or nickel-base metal parts and components for improving their erosion and high temperature corrosion resistance.
A number of processes to produce wear-resistant or corrosion-resistant surface diffusion coatings have been developed, patented, and commercialized for use in low, intermediate, and high temperature industrial applications where steel parts are subjected to significant levels of erosion and various levels of oxidation and sulfidation corrosion. Examples of these coating processes include chromizing (diffusion of chromium into the surfaces of steel components) and carburizing (diffusion of carbon into the surfaces of steel components).
Chromized coatings provide excellent protection against high temperature corrosion, especially in applications where combustion is involved, such as in boilers. Case carburizing produces hard, durable surfaces which provide protection against erosive wear, especially in applications where abrasives, such as coal, ore, or silicates, are processed.
In many industrial operations, components need to be protected from both erosion and elevated temperature corrosion. A type of coating that provides protection against hot erosive wear and corrosion is a continuous layer of chromium carbide. Although very thin chromium carbide surface layers (typically 1 mil thick or less) may be incidentally produced in the process of chromizing, such thin layers are not sufficiently durable to provide effective, long-term resistance against hot erosive wear in utility boilers. Moreover, both incidental and intentional chromium carbide layers created by current methods are often non-uniform and do not have a consistent, continuous character (instead, these layers typically have a particle-base characteristic).
It is known from the technical literature that the composition of a protective chromium carbide layer will be of the general form M23C6. Additionally, it is known that chromium carbides produced in the surfaces of carbon steels have a more complex form, (Cr, Fe)23C6. Under certain thermal processing conditions, the presence of certain carbide stabilizers in the alloy composition, such as titanium, columbium, or zirconium, may further alter the protective layer so that the layer partially consists of other carbide forms, including M3C and M7C3. Thus, the alloy composition and thermal processing conditions for a component which is to be coated with chromium carbide can have a significant effect on the form, structure, composition, and overall quality of any resulting chromium carbide coating. Notably and as above, most currently known chromium carbide layers are not continuous and, instead, are composed of individual carbide particles.
U.S. Pat. No. 5,912,050, assigned to McDermott Technology, Inc. and The Babcock and Wilcox Company, discloses an improved method for chromizing small parts in a retort. U.S. Pat. No. 5,135,777, assigned to The Babcock and Wilcox Company, discloses a method for diffusion coating a workpiece with various metals including chromium by placing ceramic fibers next to the workpiece and then heating to diffuse the diffusion coating into the workpiece. U.S. Pat. No. 5,344,502, assigned to The Babcock and Wilcox Company, discloses a method for pack carburizing certain stainless steels. All of these patents are hereby incorporated within.
The present invention produces chromium carbide coatings, greater than 5 mils thick, in the metal surface of a component and contemplates two basic methods for producing a protective chromium carbide coating in the surface through diffusion at elevated temperatures: (a) pack carburizing ferrous-base and/or nickel-base metal surfaces, followed by chromizing; and (b) chromizing metal surfaces containing higher levels of carbon (xe2x89xa70.40%C). Use of the term xe2x80x9cchromizingxe2x80x9d expressly includes co-diffusion methods known in the art. These methods successfully produce robust chromium carbide coatings (a coating with a thickness greater than 5 mils) in many steels, including T11, T22, 309 stainless steel, 310 stainless steel, 316 stainless steel, AISI 4140, AISI 4340 and UNS N06600 (a nickel-base alloy also known as Inconel 600(trademark)). Accordingly, the invention provides a feasible and commercially viable method for producing chromium carbide coatings in metal surfaces, including ferrous materials, such as carbon steels, and nickel-base alloys, such as Inconel 600(trademark).
Testing of the present invention showed the unexpected importance of the processing sequence; i.e., the necessity of having the carbon in the substrate material before chromizing, in order to form the chromium carbide coating. Specifically, it was found that chromizing the material first, followed by carburizing, would not form a useful or substantial chromium carbide coating. It is believed that the mobility and inward diffusion of carbon atoms is somehow reduced or restricted when chromium atoms are already present at some threshold concentration within the matrix, while the diffusion of chromium atoms within a matrix containing significant concentrations of carbon atoms is apparently not restricted.
The present invention comprises a method for producing chromium carbide coatings by providing a component having a metal surface, made of a ferrous-base and/or nickel-base material which includes a selected amount of carbon (i.e., alloyed or carburized to contain at least 0.40% by weight carbon) and then chromizing the surface to form a chromium carbide coating on the surface.
Another aspect of the invention further includes a method wherein the metal surface of the component is carburized, by any known carburizing method, prior to the chromizing step.
Yet another aspect of the invention further includes the application of tailored laminate coatings subsequent to the chromizing step so as to impart upon the resulting steel component a multi-layered coating with specific, desired qualities.
Accordingly, an object of the present invention is to provide a method for producing components with surfaces having a robust chromium carbide coating. Such a coating will enhance the wear and corrosion resistance of the resulting component. Furthermore, this coating is continuous and may further consist of multiple discrete layers, with each layer having its own particular morphology and concentration of chromium carbide precipitates. The continuous nature and, where applicable, layered structure of the coatings provided by the present invention further enhance its performance and durability in comparison to previous chromizing and/or carburizing methods.
Another object of the invention is to provide a method for producing components with surfaces having a tailored, multi-layered coating(s), including a base chromium carbide coating, in order to increase the resulting components"" wear and corrosion resistance (in addition to any further properties inherent to the tailored coating(s) that may be selected). The tailored, multi-layered coating includes a chromium carbide layer diffused into the surface and subsequent application of at least one additional layer selected from: titanium nitride, zirconium nitride, tantalum nitride, chromium nitride, and cobalt-tungsten carbide. This tailored coating is not necessarily diffused, but instead may reside on top of the original chromium carbide coating.
The method of applying the additional tailored layer(s) is selected according to the composition of each layer and includes: thermal spraying, physical and/or chemical vapor deposition, and sputter-ion plating. Those skilled in the art will appreciate the significance of using these specific layers, either singly or in combination, and further will understand the methods necessary to apply each additional layer(s).
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which a preferred embodiment of the invention is illustrated.